Conventional fiber optic communication systems are well-developed for transmitting high-data rate signals, such as 10 Gbps and 40 Gbps. However, rates of fiber optic communication systems are being pushed towards ever increasing speeds due to rapid growth in networks. For example, current standards bodies (e.g., IEEE) are considering data rates of 100 Gbps, which would require ever higher transmission rates (i.e., approximately 112 Gbps), once forward error correction (FEC) and framing (e.g., G.709) overheads are considered. Other standards bodies (e.g., ITU-T) are considering data rates of 120 Gbps, again requiring even higher transmission rates (i.e., approximately 130 Gbps). Such high-data rates are beyond the limit of conventional electronics and optics. For example, conventional systems utilize a direct binary modulation scheme. Disadvantageously, direct binary modulation schemes have poor spectral efficiency which limits overall transmission system utilization in wavelength division multiplexed (WDM) systems, and such schemes are difficult to implement directly with conventional electronics due to the high data rates.
Differential Quadrature Phase Shift Keying (DQPSK) can be utilized as a modulation format for improved spectral efficiency. Additionally, DQPSK provides an improved tolerance to chromatic dispersion and reduces other deleterious effects. DQPSK modulates data onto the phase of a laser in a differential way. At the demodulator, a delay line interferometer is utilized to identify the signal. Advantageously, DQPSK enables two bits to be transmitted per symbol, so a 40 Gbs or 100 Gbps data rate signal could be transmitted on a 20 Gbps or 50 Gbps carrier, respectively. Advantageously, this provides higher effective transmission rates while utilizing lower speed rates for electronic components. Also, DQPSK can be utilized with Return-to-Zero (RZ) line coding in optical transmission systems.
In RZ-DQPSK modulation, it is essential to control phase between the RZ portion of the system and the DQPSK portion of the system. DQPSK includes an in-phase signal (I data) and a quadrature signal (Q data). For example, a 100 Gbps signal is broken into two separate 50 Gbps signal streams which are referred to as the I and Q data. This is called preceding. The I and Q data are precoded from a data processor and precoder and then modulated in the DQPSK format, i.e. each of the I and Q data streams are provided to separate arms of the DQPSK modulator. Note, phase must be aligned between the I and Q data streams in the DQPSK portion.
Additionally, the RZ portion also requires phase alignment. The phase between the RZ and DQPSK portions represents an additional phase control to the system from the phase control required between the I and Q data. Accordingly, RZ-DQPSK systems require two feedback systems to process and correct phase differences including a DQPSK feedback system to control phase differences between I and Q data and an RZ portion feedback system to control phase differences between a carver clock and a DQPSK modulator output.
Referring to FIG. 1, in a conventional RZ-DQPSK system 10, a distributed feedback (DFB) laser 12 is connected to a carver modulator 14. The carver modulator 14 is modulated with an electrical clock from a clock source 16 through a voltage controlled phase shifter 18 applied to a carver driver 20. The output of the carver modulator 14 is connected to a DQPSK modulator 22. A data processor/precoder/multiplexer 24 receives an input data stream, and precodes the data stream into I and Q data which is supplied to modulator drivers 26,28. The output of the DQPSK modulator 22 includes a tap to provide a portion of the output to a monitor and control circuit 30. For example, the tap splits a small portion (e.g., 5%) of the optical signal for monitoring. The monitor and control circuit 30 is configured to detect the signal and the associated phase differences between I and Q data and the associated phase differences between the carver clock 10 and the DQPSK modulator 22 output.
The control circuit 30 provides a control signal to the data processor/precoder/multiplexer 24 to adjust phase between data fed to the I and Q arms of the modulator drivers 26,28. The control circuit 30 provides another control signal to a phase controller 32 to adjust phase between the carver modulator 14 and the DQPSK modulator 22. The phase controller 32 connects to the phase shifter 18 to provide a reference signal. The control circuit 30 is operable to provide continuous correction of both the phase between I and Q arms of the modulator drivers 26,28 and between the carver modulator 14 and the DQPSK modulator 22. Additionally, the carver modulator 14 together with the clock source 16 can be located after the DQPSK modulator 22. Here, the DFB laser 12 is directly connected to the DQPSK modulator 22.
Disadvantageously, existing RZ-DQPSK systems 10 require two separate phase control loops resulting in additional circuitry, complexity, and cost. Additionally, existing RZ-DQPSK systems 10 require the carver modulator 14. It would be advantageous to provide single real-time phase alignment systems and methods for all phase alignments to reduce components, cost, and space in RZ-DQPSK systems 10. Further, it would be advantageous to remove the need for a separate carver modulator 14 in RZ-DQPSK systems 10.